Deep Absorbing Porphyrin Small Molecule for High

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Deep Absorbing Porphyrin Small Molecule for High Performance Organic Solar Cells with very Low Energy Losses Ke Gao, Lisheng Li, Tianqi Lai, Liangang Xiao, Yuan Huang, Fei Huang, Junbiao Peng, Yong Cao, Feng Liu, Thomas P. Russell, René A. J. Janssen, and Xiaobin Peng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Jun 2015 Downloaded from http://pubs.acs.org on June 2, 2015

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Journal of the American Chemical Society

Deep Absorbing Porphyrin Small Molecule for High Performance Organic Solar Cells with very Low Energy Losses Ke Gao,† Lisheng Li,† Tianqi Lai,† Liangang Xiao,† Yuan Huang,† Fei Huang,† Junbiao Peng,† Yong Cao,† Feng Liu,*,‡ Thomas P. Russell,‡,¶ René A. J. Janssen,*, § and Xiaobin Peng*,† †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, P. R. China; ‡Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA, 94720, USA; ¶Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003, USA; §Molecular Materials and Nanosystems, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

Supporting Information Placeholder which photons are converted into collectable charges,11 and therefore can lead to high performance photovoltaic devices. For BHJ OSCs, the minimum Eloss was suggested to be 0.6 eV,11 and the corresponding Voc is often used as the achievable maximum Voc. However, the Eloss values below 0.6 eV are still rare even for PSCs 13,14 and no such report for small molecule-based OSCs. It also should be noted that, though low energy losses below 0.6 eV were sometimes reported for PSCs, their performance often dropped dramatically when reducing Eloss to below 0.6 eV.13 For a small molecule, the limited backbone length restricts linear extension of conjugation, which retards absorption in the longer wavelength region. Strong donor-acceptor pairs can extend absorption but the energy level offsets sacrifice Voc. 15-17 To date, the most efficient small molecules were built on oligothiophenes, which showed limited absorption but high Voc values, 4,18-21 or donor-acceptor hybrids with wider absorption and larger current but limited Voc. 22-32 Extracting more energy from bulk heterojunction blends requires simultaneously enhanced photocurrent and voltage, which rely on the further optimization of both electron donating materials and the BHJ morphology. New material design of both polymer and small molecules that can extend photon absorption to over 800 nm, as well as maintaining a high Voc and a high PCE, will be the next major advance in OSCs either with single junction or tandem structures. Porphyrin analogues have extensive π-conjugated surface and show high efficiency in photosynthesis.33 In this contribution, two diketopyrrolopyrrole (DPP) units are linked to the both sides of a porphyrin core to extend the π-conjugation via ethynylene bridge, whose functions are two-fold 34,35 (1) introducing sp1 carbon hybridization can be effective in lowering the highest occupied molecular orbital (HOMO) energy level of the target materials since there are more s-orbital components; and (2) its fluent cylinderlike π-electron density is more adaptable to conformational and steric constrains, and thus to enhance inter-molecular π-π stacking and facilitate intramolecular charge transport. Since it has been reported that thienyl substituents can enhance the OSC’s performance, two thienyl groups with a 2-ethylhexyl substituent are introduced into the porphyrin core with added benefit of improving material’s solubility. The target molecule 5,15-bis(2,5-bis-(2-ethylhexyl)-3,6-dithienyl-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-5’-ylethynyl)-10,20-bis(5-(2-ethylhexyl)-thienyl)-porphyrin zinc(II)

ABSTRACT: We designed and synthesized DPPEZnP-TEH molecule, in which a porphyrin ring was linked to two diketopyrrolopyrrole units by ethynylene bridges. The resulted material exhibited a very low energy band gap of 1.37 eV and a broad light absorption to 907 nm. An open circuit voltage of 0.78 V was obtained in bulk heterojunction (BHJ) organic solar cells, showing a low energy loss of only 0.59 eV, which is the first report that small molecule solar cells show energy losses below 0.6 eV. The optimized solar cells show remarkable external quantum efficiency, short circuit current, and power conversion efficiency values up to 65%, 16.76 mA/cm2, and 8.08%, respectively, which are the best values for BHJ solar cells with very low energy losses. Besides device optimization, the morphology of DPPEZnP-TEH neat and blend films with PC61BM was studied thoroughly by grazing incidence x-ray diffraction, resonant soft x-ray scattering and transmission electron microscopy under different fabrication conditions.

Solution processed organic solar cell (OSC) is a promising renewable energy source that has attracted much interest recently.1,2 The power conversion efficiencies (PCEs) of OSCs have surpassed the 10% barrier for single junction devices and are expected to be higher for tandem devices. 3-7 In comparison to polymeric materials, small molecules (SMs) have the advantages of well-defined structures and molecular weights. Besides, SMs are easy to purify, and have good batch-to-batch reproducibility, which have enriched the material choices for OSC commercialization.8-10 In order to enhance the performance of OSCs, chemists in photovoltaics have been trying to extend the material’s absorption to long wavelength over 800 nm (namely very low energy band gap Eg) to absorb more light. However, it is still difficult to design high efficient organic materials including polymer and small molecule donors that show very low energy band-gaps and high open circuit voltages (Voc) for OSCs simultaneously. The energy loss (Eloss), which connects Eg and Voc together and was defined as Eloss = Eg− eVoc, is one of the important parameters to evaluate solar cells. 11 Low Eloss values, which are less than 0.5 eV in high efficient perovskite solar cells and 0.34–0.48 eV for the best inorganic crystalline solar cells, 12 enhance the energy efficiency by

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(DPPEZnP-TEH) was synthesized by Sonogashira coupling reaction (ESI), affording 75% of the final product, with added benefits of avoiding toxic stannyl intermediates and the potential chemical accidents from lithiation reactions in the whole synthetic routes. The absorption spectra of DPPEZnP-TEH in solution and in thin film showed a broad spectral coverage in the visible and near infrared regions (Figure S3). In very dilute dichloromethane solution, three strong absorption peaks at 466, 568 and 727 nm were observed. Such strong absorption in the near infrared region at a short backbone length indicates that the π-electron delocalization between constituent units is very effective. This is due, partially to the ethynylene bridge, and partially to the large aromatic surface that extends the conjugation. In thin film, DPPEZnP-TEH exhibited red-shifted absorption peaks at 472, 573, and 802 nm with a shoulder peak at 739 nm. The significant red-shift of 75 nm from 727 nm in solution to 802 nm in film is attributed to the strong intermolecular π–π stacking in solid state, possibly due to the optically active J aggregation.36-38 The optical band gap of the solid thin film is calculated to be 1.37 eV based on the onset of the absorption spectra (907 nm). Thermal annealing was applied to DPPEZnP-TEH thin film (Figure 1a). It was seen that the intensity of 802 nm peak decreased under thermal treatment and the peak center slightly shifted to the shorter wavelength region (786 nm). Annealing also induced a new peak at 720 nm, which intensified under elevated temperature. This new optical state possibly came from Haggregation of DPPEZnP-TEH molecules, which was formed, more than likely, by sliding the intermolecular π-π stacking, changing from J- to H-aggregation. The absorption spectrum of DPPEZnP-TEH / with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) (1:1.2, w/w) blend thin films was also investigated. Chlorobenzene (CB) -cast thin films showed a dominant peak at 798 nm with a shoulder peak at 732 nm (Figure 1b). Under thermal annealing, the 798 nm peak decreased and blue-shifted to 786 nm, and the shoulder peak at 732 nm increased and blue-shifted to 722 nm, which was the same trend as pure DPPEZnP-TEH film. A more significant reduction of the 798 nm peak to 783 nm and increase of the 722 nm peak to 718 nm were observed for the blend films with PC61BM prepared in the presence of pyridine additive and then under thermal annealing. Cyclic voltammetry (CV) measurements were carried out to estimate the HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of DPPEZnP-TEH. As shown in Figure S4, the onset oxidation (Eox) and reduction (Ered) potentials of DPPEZnP-TEH were 0.82 and -0.56 V vs. Ag/AgCl electrode, respectively, and the formal potential of Fc/Fc+ vs. Ag/AgCl electrode was recorded to be 0.48 V in this work, from which the HOMO and the LUMO energy levels were estimated to be -5.14 and -3.76 eV according to the empirical formula of EHOMO = -e (Eox + 4.32) (eV) and ELUMO = -e (Ered + 4.32) (eV), respectively. 39 The electrochemical energy band-gap was 1.38 eV, which was almost the same as the optical energy gap calculated from the onset of the absorption spectrum in film.

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The solution-processed BHJ OSCs were fabricated utilizing PC61BM as the electron acceptor and DPPEZnP-TEH as the electron donor under a conventional device structure of ITO/PEDOT:PSS/active layer/PFN/Al (ITO: indium tin oxide, PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), PFN: poly[(9,9-bis(30-(N,N-dimethylamino)-propyl)2,7-fluorene)]-alt-2,7-(9,9-dioctylfluorene), and then measured under AM 1.5 illumination. The ratio of DPPEZnP-TEH and PC61BM was optimized to be 1:1.2 (w/w). Device results are summarized in Figure 2 and Table 1. An exceptionally high Voc of 0.78 V was obtained for the devices processed under different conditions, indicating a suitable alignment of the HOMODPPEZnPTEH and LUMOPCBM. It is worth commenting that the photon energy loss Eloss (Eloss = Eg− eVoc ) is only 0.59 eV, which is the first report for small molecule-based BHJ OSCs, and Eloss values below 0.6 eV are still rare even for PSCs. Since the minimum Eloss has previously been suggested to be 0.6 eV and the corresponding Voc is also often used as the maximum achievable in BHJ OSCs, the 0.59 eV energy loss of DPPEZnP-TEH:PC61BM OSCs is very low and thus the Voc of 0.78 V is very high for low energy band gap materials of only 1.37 eV. The BHJ devices fabricated without solvent additive and without thermal annealing exhibited a good power conversion efficiency (PCE) of 5.53% with a short circuit current (Jsc) of 12.72 and a fill factor (FF) of 55.76%. When the OSCs were thermally annealed at 120 °C for 10 min, only slightly performance enhancement of a FF to 59.9% and a PCE to 5.89% was observed. The devices fabricated in the presence of 1% pyridine additive and then thermally annealed showed drastic enhancement in PCE to 8.08%, Jsc to 16.76 mA/cm2, and FF to 61.8%, at an almost unchanged Voc of 0.78 V. The Jsc value of 16.76 mA/cm2 is the highest for small molecule-based solar cells, and the PCE up to 8.08% makes DPPEZnP-TEH one of the top-tier light-harvesting molecules.

Table 1. Summary of device performances Devices

Jsc (mA/cm2)

Voc (V)

FF (%)

PCEa) (%)

as cast

12.72

0.78

55.76

5.53(5.47)

annealed

12.44

0.79

59.90

5.89 (5.82)

Py-annealed

16.76

0.78

61.80

8.08 (8.04)

a)

The values in square brackets are the average PCE obtained from 20 devices. Compared to the alkoxy phenyl groups we reported previously, the effects of the alkythienyl groups are twofold: (1) deepening the EHOMO, which is demonstrated by density functional theory optimizations (ESI), electrochemical and absorption measurements and the slightly higher VOC; and (2) changing from J- to Haggregation when the films are prepared in the presence of pyridine additive then thermal annealed. And it was reported that the ratio of the H-aggregate should be considerably larger than the Jaggregate to increase cell performance.40 The external quantum efficiencies (EQEs) of the devices were measured. A broad-spectrum response from 380 to 907 nm was observed. Chlorobenzene processed devices showed EQEs of ~45% over a broad region. No large differences were seen with thermally annealed devices. However, pyridine additive significantly elevated the EQE to 65%. Single carrier devices were also fabricated. The hole-mobilities of the blend and pure DPPEZnPTEH films were calculated to be 4.85×10-4 and 1.12×10-3 cm2V−1s−1, respectively, by the space charge limited current (SCLC) method,41 which is similar to other of high performance systems.20

Figure 1. UV-vis-NIR absorption spectra of DPPEZnP-TEH thin films (a) and BHJ films (b).


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Journal of the American Chemical Society this transformation can be a consequence of the large aromatic surface, which offers more contact area and more relative positions of stacked porphyrins that could serve as channels for carrier transport that maintain high device performance.

Figure 2. J–V curves (a) and EQE (b) of the OSCs using DPPEZnP-TEH:PC61BM (1:1.2, w/w) active layer. It should be noted that, though low energy loss for OSCs below 0.6 eV was reported for PSCs, the quantum yield for charge generation often drops dramatically for Eloss < 0.5 eV.11,35 In current case, the photovoltaic parameters of EQE, Jsc, and PCE are kept at a very high level simultaneously, which are all the highest values within the 0.6 eV Eloss barrier, indicating the possibility of synergic material design to simultaneously enhance performances. There are several conditions for reducing the energy loss and obtain high VOC.42,43 The permittivity of DPPEZnP-TEH is 3.9 at 10 kHz, which is larger than the values of the three well-known OPV materials (ESI) and similar with that of the previously synthesized material,28 suggesting that the large permittivity can contribute to the very low Eloss of the porphyrin-based OSCs.42 However, the free base porphyrin (DPPEH2P-TEH, ESI) shows a permittivity of only 3.1, and its optical band-gap is 1.32 eV. The OSCs based on DPPEH2P-TEH as the donor and PC61BM as the acceptor exhibit a PCE of only 2.52% with a VOC of 0.74 V (ESI), and their Eloss of 0.58 eV is the same as that of DPPEZnP-TEH. These results indicate that that the low Eloss values for these OSCs can be closely related to the molecular constitution of the conjugated plane, and the central Zn (II) in DPPEZnP-TEH help improving the fill factor and current of OSCs devices, which could be also a consequence of morphology improvement. Different processing conditions can lead to variations in Jsc and FF, and an improved morphology usually gives rise to enhanced device performance, pointing out the importance of morphology optimization in OSCs. The morphology properties of DPPEZnPTEH in pure and blend films with PC61BM were characterized by grazing incidence x-ray diffraction (GIXD) (Figure 3). In pure film, a weak diffraction ring at ~0.32 Å-1 (1.96 nm) and a broad ring at ~ 1.48 Å -1 (0.42 nm) were observed. The low q diffraction peak arose from the alkyl-to-alkyl distance or (100) reflection; while the reflection at high q diffraction arose from average π-π distance. The crystallites did not shown strong orientation inside the film, as evidenced by the azimuthal spreading of the (010) diffraction peak, and specular reflection distorted (100) peak diffraction thus cannot be used to access crystal orientation. Thermal annealing led to a slight decrease of the (100) peak area and a shifting of peak position to 0.30 Å-1. In blends processed from CB, a better-defined (100) diffraction peak was observed at 0.27 Å-1 (2.32 nm) with a “crystal size” of 8.6 nm (corresponding to 34 stacks). A weak diffraction peak arising from the PCBM was seen at 0.68 Å-1, corresponding to a distance of 0.92 nm. The broad reflection at ~1.49 Å-1 corresponds to the π-π stacking peak of DPPEZnP-TEH (overlapping with PCBM diffraction). The CB+Py processed thin film blends showed a more obvious (100) diffraction at 0.29 Å-1 (2.17 nm). The coherence length was estimated to be 6.6 nm, corresponding to three stacks. The (100) peak area was slightly larger for CB+Py processed blends, indicating a better crystallinity. The combined GIXD and absorption investigation indicated that pyridine additive would lead to enhanced crystallinity in (100) direction with reduced crystal size. In π-π stacking direction, the slipped J-type molecular packing was transformed to a less slipped H-type aggregation,38 in which the porphyrin-porphyrin interaction was enhanced. The flexibility in

Figure 3. GIXD diffractogram (a) and out-of-plane line-cut profiles (b) of DPPEZnP-TEH pure and blend thin films. The morphology of the BHJ thin film was characterized by transmission electron microscopy (TEM). In TEM, CB processed thin films showed an obvious phase separation with an average center-to-center distance of ~100 nm. The black and white domains were PCBM rich and DPPEZnP-TEH rich regions, respectively. No strong PCBM aggregation was seen in this sample, and the device had a Jsc of 12.72 mA/cm2. Thermal annealing of this sample led to a blurring of the phase image, however the size scale of phase separation remained similar. Pyridine was a quite effective additive in enhancing the order along the (100) direction in this system. 1% Pyridine in solution strongly changed the morphology of the resulting thin film. As shown in Figure 4a, a much finer phase separation was observed, with a characteristic size scale of ~30 nm. This sharp reduction in length scale led to a 35% increase in photocurrent, reaching a value of 16.76 mA/cm2.

Figure 4. TEM (a) and RSoXS (b) of DPPEZnP-TEH:PCBM blends processed from different conditions. Resonant soft x-ray scattering (RSoXS) was used to further confirm the morphology of these thin films. A carbon K-edge scattering was taken for each sample and results were shown in Figure 4b. CB processed thin film (annealed) showed a strong diffraction peak at 0.0066 Å-1, corresponding to a distance of 95.2 nm; CB+Py processed thin film (annealed) showed a broad peak at 0.0221 Å-1, corresponding to a distance of 28.4 nm. The RSoXS results were in good agreement with the TEM observations. It is worthy to note that the specific inner surface area (donor-acceptor interfaces) is inversely proportional to the length scale of phase separation, and thus CB+Py processed thin film will have a roughly two time larger inner surface area. However only 35% Jsc increase was seen. The slow decay of the RSoXS profile for films prepared from CB hindered the observation of phase separated domains of smaller length-scales (see peak fitting in supporting



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information), which enhanced light extraction in blended thin film and enhances photocurrent in CB processed devices. The surface morphology of BHJ thin films were probed by atomic force microscopy (AFM) and it was seen that pyridine additive processed thin film showed a smoother surface, with a RMS roughness of only 0.52 nm (Figure S15). In summary, a new conjugated DPP-porphyrin small molecule with 2-ethylhexyl-thienyl side groups at the porphyrin core was developed. The BHJ OSCs gave us high EQE, Jsc, FF and PCE up to 65%, 16.76 mA/cm2, 61.8% and 8.08%, respectively, with very low energy loss of only 0.59 eV. The combination of high permittivity, main chain conjugation and good morphology delivers a good EQE and low energy loss. The central Zn(II) also plays an important role for the high PCEs of the low energy loss OSCs. Successful demonstration of this porphyrin for OSCs suggests a new methodology in material design to achieve high performance OSCs with very low energy loss, and its superior performance will also spur further research in more complicated device settings.

ASSOCIATED CONTENT Supporting Information Experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the grants from International Science & Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), the Ministry of Science and Technology (2014CB643500), and the National Natural Science Foundation of China (51473053, 51073060). FL and TPR was supported by Polymer-Based Materials for Harvesting Solar Energy (PHaSE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under award number DE-SC0001087. Portions of this research were carried out at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source, and Molecular Foundary, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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